1. Field of the Invention
The present invention relates to a curable composition capable of being cured with high sensitivity by energy application such as laser exposure and to a negative-working planographic printing plate precursor with a photosensitive layer to which the curable composition has been applied, on which precursor images can be directly drawn with high sensitivity by laser light.
2. Description of the Related Art
Conventionally, a PS plate having a lipophilic photosensitive resin layer provided on a hydrophilic support has been widely used as a planographic printing plate precursor, and a desired printing plate is obtained by a plate-making method which usually involves masked exposure (surface exposure) via a lithographic film and then removing non-image regions by dissolution. In recent years, digitalization techniques which involve electronic processing, accumulation and output of image information with a computer have been spreading. A wide variety of new image output systems compatible with such digitalization techniques have come to be practically used. As a result, there has been demand for computer-to-plate (CTP) techniques of producing a printing plate directly by scanning a highly directional light such as laser light according to digitalized image information without a lithographic film. It has been a critical technical issue to provide a planographic printing plate precursor suitable for these techniques.
As a negative-working planographic printing plate precursor capable of such scanning exposure to light, a planographic printing plate precursor has been proposed which includes a photopolymerizable photosensitive layer including a photopolymerization initiator, an addition polymerizable ethylenically unsaturated compound and a binder polymer having a specifically structured repeating unit which is soluble in an alkali developing solution and, according to necessity, a oxygen-impermeable protective layer, provided on a hydrophilic support (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-318053). In addition, a negative-working planographic printing plate precursor has been proposed which includes a photopolymerizable photosensitive layer and an oxygen-impermeable protective layer containing an inorganic laminar compound, provided on a hydrophilic support (see, for example, JP-A No. 11-38633).
In a negative-working planographic printing plate precursor having a photopolymerization curable photosensitive layer, a radical polymerization reaction is inhibited in the presence of oxygen, and therefore problems such as lowering of sensitivity and variation in sensitivity due to oxygen concentration variation easily occur. For preventing such problems, it is common to form an oxygen-impermeable protective layer coated on the surface as described above or to coat the surface of the photosensitive layer with a cover sheet. However, the formation of a protective layer or cover sheet requires processing steps, and the protective layer or cover sheet must be removed at the time of plate making using the planographic printing plate precursor. Therefore, development of negative-working photosensitive compositions which require no protective layer due to increased sensitivity of a photosensitive layer has been attempted. For example, photosensitive compositions containing a binder having an ether linkage (see, for example, JP-A No. 8-160612) and photosensitive compositions containing a styrene binder (see, for example, JP-A No. 2001-290271) have been proposed. However, a problem with the photosensitive compositions containing a binder having an ether linkage is that a sensitivity high enough for practical use cannot be obtained. A problem with the photosensitive composition containing a styrene binder is that although the sensitivity is successfully improved, they have low solubilities in alkali developing solutions or solvents and therefore development operations cannot be performed efficiently.
For this reason, there has been a demand for curable compositions suitable for a photosensitive layer of a negative-working planographic printing plate precursor which can be cured at high sensitivity by light exposure, cause less sensitivity variation during storage, and are excellent in developability.